The present state-of-the-art in semiconductor radiation detection is silicon diodes, high purity germanium (cooled by liquid nitrogen), and compound semiconductors, such as cadmium zinc telluride (CZT) and mercuric iodide. Each of these materials has one or more significant drawbacks related to its use. Silicon has a low atomic number and is therefore primarily useful for the detection of energetic charged particles and atomic x-rays emitted by low atomic number elements. Germanium has a higher atomic number but, because of its low band gap energy, must be cooled by liquid nitrogen in a bulky, expensive, and potentially dangerous cryogenic system in order to reduce thermally generated noise. Compound semiconductors, such as CZT and mercuric iodide, have sufficiently high band gap energy to be useful at or near room temperature. However, CZT has been plagued by production problems, resulting in polycrystalline ingots with twins, inclusions, and grain boundary defects. These defects can never be completely removed and are a consequence of CZT being a solid solution, rather than a true compound. The result is that spectroscopy grade crystals must be mined from bulk material. Mercuric iodide suffers from low carrier mobility, short carrier lifetime, space charge polarization, and surface degradation. In addition, mercuric iodide is an extremely soft material that is easily damaged by the slight pressure of an electrical connection and by temperatures over sixty degrees Celsius. In general, these compound semiconductors do not interact with neutrons such that they must be coupled with a thin layer of a neutron absorbing material, such as 6LiF or 10B. A reaction between 6Li or 10B occurs in the thin absorber layer, which creates alpha particles that are detected by a semiconducting substrate. The absorber layer must be thin in order for the semiconducting substrate to detect the resultant alpha particles. 3He gas filled tube detectors are the state-of-the-art for thermal neutron detection.
As a result, U.S. Pat. No. 7,687,780 (Bell et al.) provides a semiconductor detector of ionizing electromagnetic radiation, neutrons, and energetic charged particles. The detecting element includes a compound having the composition I-III-VI2 or II-IV-V2, where the “I” component is from column 1A or 1B of the periodic table, the “II” component is from column 2B of the periodic table, the “III” component is from column 3A of the periodic table, the “IV” component is from column 4A of the periodic table, the “V” component is from column 5A of the periodic table, and the “VI” component is from column 6A of the periodic table. The detecting element detects ionizing electromagnetic radiation by generating a signal proportional to the energy deposited in the element, and detects neutrons by virtue of the ionizing electromagnetic radiation emitted by one or more of the constituent materials subsequent to capture. The detector may contain more than one neutron sensitive component.
Related to the I-III-VI2 compounds, however, improved methods for combining the elemental constituents in a multistep synthetic process are still required, providing improved purity and homogeneity and more precisely controlling the reaction rate and yielding a I-III-VI2 charge with a single phase stoichiometry.
Further, large arrays of helium-3 gas tubes are currently being used in neutron imaging applications. These neutron imagers can be designed to allow a wide range of incident neutron energies in samples and, as such, require a similarly wide range of neutron energies to be detected. Current and future usage forecasts suggest that helium-3 supplies will not be sufficient to sustain worldwide neutron detection needs, thereby creating a need for alternative materials. LiF-coated semiconductors and scintillators are the leading candidates; however, their neutron efficiency is limited to less than about 20%. Thus, what is still needed in the art is an alternative material that provides much greater neutron efficiency, enhanced sensitivity, enhanced spatial resolution, smaller detector footprint, and lower cost. It should be noted that, thermal and cold neutron imaging is typically conducted at neutron science centers and the like, while fast neutron imaging (i.e. radiography) is typical of nonproliferation and treaty verification and the like.